is it possible to implement Adwords conversion tracking so that it takes into account conversion that come through payment portal (Customer leaves site for a moment and returns after payment, usually conversion is not tracked due leaving site for a moment)?
I have injected adwords tracking code to order completion page but it tracks only orders that come with billing option (customer does not leave the site).
If this is not possible, is it possible to inject code to order review page, if so how? (This is bad as some customers wont complete order so leads to false conversions. Also we are planning skipping order review page for billing)
You have to make users come back to your site adding this query string in the URL:
Then Google will discard referral modification caused by leaving the site.
See this question.
You will have to deal with your payment gateway to modify the return URL, I think.